1. Field of the Invention
The present invention relates to filter devices.
2. Description of the Related Art
To date, filter devices that utilize elastic waves such as surface acoustic waves or boundary acoustic waves have been mounted in communication devices such as cellular phones. For example, in Japanese Unexamined Patent Application Publication No. 62-261211, a filter device is described in which a loosely coupled transformer is connected in parallel with a main filter and a signal having a same level as and a phase opposite to a direct wave mixes with and cancels out the direct wave. In this filter device, a direct wave is canceled out and therefore attenuation in a stopband can be made large.
In recent years, the demand for further decreases in the size of filter devices has been increasing without a solution.